Light has been used to disinfect pathogens on surfaces, in air, or in water (i.e. to inactivate pathogens in fluids, or on solid surfaces). Herein the term “pathogen” refers to any microscopic organism capable of causing disease or infection in a human. These include bacteria, viruses, spores, and fungi. Herein the term “inactivate” refers to rendering a pathogen inactive, or unable to infect a human. This may include killing the pathogen, rendering it unable or less able to replicate, or rendering it unable to infect a human. Some known systems use ultraviolet (UV) light to inactivate pathogens. Herein, we adopt the standard definitions, as follows: ultraviolet light refers to light having wavelengths in the range 100 nanometers (nm) to 400 nm; the four sub-ranges within the UV range include the Vacuum UV from 100 to 200 nm; UVC from 200 to 280 nm; UVB from 280 to 315 nm; and UVA from 315 to 400 nm. Although the visible colors are not rigorously defined in the lighting industry, herein we adopt commonly used definitions for violet and blue as comprising about 400 to about 450 nm, and about 450 to about 490 nm, respectively. Some known systems use UV light in the range of 200 to 300, including the UVC range and some of the UVB range, to inactivate pathogens by damaging their DNA or RNA and rendering them incapable of reproduction, thus incapable of causing disease in humans. This range of 200 to 300 nm is referred to in the literature, and herein, as the germicidal range. Light sources such as low-pressure and medium-pressure mercury lamps, and pulsed xenon lamps are known to inactivate pathogens by irradiation of fluids (e.g. water or air or other fluids) and surfaces with wavelengths in the germicidal range.
A description, with references, of disinfection lighting using the germicidal range of light is found in a recent publication “Light based anti-infectives: ultraviolet C irradiation, photodynamic therapy, blue light, and beyond”, Yin et al., Curr Opin Pharmacol, 2013 October. Light of these wavelengths can have high inactivation rates for many types of pathogens on surfaces, in air, or in water. But, exposure to light of these germicidal wavelengths can be hazardous to human beings. As a result, these systems may only be used safely in locations where human beings are not present, or are prevented from accessing.
Other systems may use violet, blue, or longer wavelengths of visible light to inactivate most common pathogens, but the inactivation rates of the visible wavelengths have been found to be three to five orders of magnitude lower than for the germicidal range of wavelengths of light.
U.S. Pat. No. 8,398,264 describes a lighting device that emits visible light at a wavelength and irradiance sufficient to inactivate one or more pathogenic bacterial species. U.S. Pat. No. 9,039,966 describes a method wherein the visible light for inactivating the MRSA pathogen includes wavelengths in the range of 400-420 nm, i.e., violet light. But these visible light, especially violet light, systems have several problems: disinfecting pathogens with visible light requires a very large flux density of light (e.g. about 0.5 to about 5 W/m2) incident for several hours on the surface to be disinfected; if violet light is used for disinfection, the amount of electrical power required to operate the visible LEDs at sufficient dose to inactivate about 90-99% of a population of common pathogens is so high that the overall efficacy of the lighting system is significantly reduced, by as much as about 10% to about 50% or more; if violet or blue light is used for disinfection, the flux of violet light in the space occupied by humans is so large that some occupants suffer eyestrain, headaches, nausea, dizziness or discomfort; if violet or blue light is used for disinfection, the flux of violet or blue light is so large that it greatly distorts the color point of the white light with which it might be mixed, and is so large that the flux may not be substantially increased for the benefit of more effective disinfection without exceeding the permissible limit of the blue light photobiological hazard standard, rendering the light source unsafe for humans. The limited magnitude of disinfection is a problematic limitation of violet light. The 90-99% inactivation is typically achieved only under certain favorable conditions for disinfection of an architectural space, including the following factors: vegetative bacteria, possibly excluding spores and viruses; in direct line-of-sight of, and in sufficient proximity to the disinfecting light source; and absence of biofilm; with significantly lower inactivation rates for spores and viruses. In most non-ideal circumstances, the inactivation rate may be considerably less than about 90-99%, and may therefore be ineffective, e.g. under circumstances of lower flux levels due to shadowing or distance from the disinfecting light source; biofilm or high bio-burden; spores, or viruses. As a result, these systems are expensive, energy-inefficient, visually obtrusive, physiologically disturbing to some individuals, marginally safe for human exposure, and limited in the magnitude of disinfection by the compromises in system design required to overcome these problems. The term “common pathogen” herein refers to a pathogen that is commonly responsible for human disease, especially in the context of the most commonly encountered nosocomial infections, so-called hospital acquired infections (HAI), including the well-known pathogens Staphylococcus aureus (S. aureus); Methicillin-resistant Staphylococcus aureus (MRSA); Clostridium difficile (C. diff.); Escherichia coli (E. coli.); and several other gram-positive, gram-negative, spore, viral, and fungal pathogens.
Other systems may use visible light having wavelengths centered on 405 nm light to provide inactivation of about 90-99% of a pathogen population for many common pathogens, but only if the light source generates the disinfecting light for extended periods of time (i.e., five to ten hours or more of exposure time), and if the disinfecting light is generated at significantly large radiant power densities. The dose of light for inactivation of about 90% of a population of common pathogens using 405 nm light is typically about 10-20 J/cm2, per the reference Maclean et al., High-Intensity Narrow-Spectrum Light Inactivation And Wavelength Sensitivity Of Staphylococcus Aureus, FEMS Microbiol Lett 285 (2008) 227-232. This corresponds to an irradiance of 3.5-7 W/m2 of 405 nm light, for an exposure time of 8 hours. Given the typical efficiency of 405 nm LEDs today of about 20-30% (efficiency of converting electrical power to radiated optical power) the disinfection lighting requires an electrical power density of about 12-35 Wel/m2. The electrical power density used for general white-light illumination at a level of 500 lux from a light source having a typical efficacy of 100 LPW is about 5 Wel/m2. If the disinfection lighting, providing about 90-99% disinfection on a target surface is added to, and mixed with, the white lighting having a flux density of about 500 lux on the target surface, then the electrical power density of the combined lighting system will be about 17-40 Wel/m2 Since the electrical power density required for disinfection using 405 nm light is much greater than that required for white-light illumination, the overall system efficacy of the illuminating and disinfecting lighting system may be reduced by as much as about 70-90%, from typically 100 LPW to about 10-30 LPW.
The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Energy Standard 90.1-2013 provides an upper limit for the Lighting Power Density (LPD) in the range of about 0.5 to about 2.0 Wel/ft2, or about 5 to about 20 Wel/m2 in typical indoor lighting applications. Specifically, for a hospital, the upper limit LPD allowed is 1.05 Wel/ft2, or 11.3 Wel/m2, calculated by the Building Area Method. For the typical values given above for a disinfection lighting system using 405 nm light, the LPD of about 17-40 Wel/m2 exceeds the ASHRAE limit of 11.3 Wel/m2. The ASHRAE limit would also constrain the flux of the disinfection portion of the lighting (the 405 nm radiation) to no more than about 6 Wel/m2 which is insufficient to inactivate 90-99% of pathogens over a period of 8 hours using 405 nm light. The ASHRAE limits may adversely affect the ability of customers to use 405 nm disinfection lighting in some regulated applications.
United States Patent Application No. US2011/0251657 A1 describes a lighting device that emits visible light and UVA light in the range 320-380 nm at an irradiance that is in the range of 3 to 15% of the irradiance of the visible light, wherein the visible light provides 700 lux at the work surface, sufficient to activate the human serotonin nervous system with the advantage of decreasing aggressiveness in humans. When the radiant energy of the near ultraviolet radiation with a wavelength of 320 nm or longer, but shorter than 380 nm, is less than 3% of the radiant energy of the visible light, advantageous effects on the serotonin nervous system would not be obtained. Since the visible light used provided 700 lux, and since 3% or more of the radiant energy in the UVA is required to activate the serotonin nervous system, then it may be expected that about 4% or more of the radiant energy should be emitted in the UVA if the visible light component is only 500 lux, instead of 700 lux in order to activate the human serotonin nervous system.